Other measuring equipment is routinely tested for accuracy and reproducibility before being used, but disposable pipettes
are rarely checked. In one case, a laboratory was checking its spectrophotometer by measuring the absorbance and the wavelength
of maximum absorbance of a carefully prepared solution of an inorganic salt. The linearity of the instrument at the maximum
wavelength was checked by making a series of dilutions using a disposable 5 mL pipette and several disposable 1 mL serological
pipettes. A plot of the results showed that the spectrophotometer was more or less linear, but the points did not fall along
a straight line. Statistical calculations showed that, while linear, the readings were highly variable. The samples were read
on a second spectrophotometer that confirmed the erratic readings. The serological pipettes that were used to prepare the
dilutions were rinsed and used to pipette known volumes of water which were weighed. The results showed that far from pipetting
a reproducible volume of 0.10 mL, the pipettes were delivering volumes ranging from 0.085 mL to 0.17 mL.
Another area of concern has to do with the composition of the disposables. Are they really made from the materials that are
stated on the labels? Does the laboratory really know what materials are acceptable for its work? When glass vials are used
as containers for products, especially parenterals, there are QC tests that must be done to verify the type of glass being
used. For example, the United States Pharmacopeia contains tests for the acceptability of borosilicate and flint glass. There are situations where the properties of the types
of glass are very important; what about disposable beakers and sample bottles?
Analysts and biochemists who deal with blood coagulation factors and membrane-bound proteins and enzymes are familiar with
problems caused by surfaces that activate or adsorb material and produce changes in the apparent potency of preparations.
Certain types of proteins are known to bind to polymers such as agarose, cellulose, and polyamides. Thus the composition of
microtiter plates, pipettes, and test tubes could affect the outcome of tests.
An assay is often validated using disposables with a given composition. If the composition changes, there may be subtle, but
real changes in the properties of the assay. Changes of this type are usually assumed not to affect an assay, and careful
checking is not done.
Some people will now wave their hands about and insist that these cautionary tales are exaggerations of highly infrequent
events. These people are certainly entitled to their opinions, but they are exactly that—opinions.
Years ago there was a commercial for a national, fast-food, hamburger chain in which an elderly woman asked repeatedly
and querulously, "Where's the beef?"
In a GMP environment, the question becomes, "Where's the proof?"
Steven Kuwahara, PhD, is the principal consultant at GXP BioTechnology LLC, PMB 506, 1669-2 Hollenbeck Ave., Sunnyvale, CA 94087; Tel 408.530.9338,